Conventionally, developers generally used in electrophotography can have a desired flowability and charging characteristics by attaching external additives on the surfaces of colored resin particles. As external additives, inorganic particles and organic particles are generally used.
Conventionally as the external additives, metal oxide particles, resin particles and these materials subjected to a surface treatment have been widely used. Among them, particularly, particles of metal oxide such as silica, titania and alumina, and these materials subjected to a hydrophobicity-imparting treatment are often used.
Silica fine particles having various particle diameters are generally used as external additives, while many inventions using alumina fine particle are also known.
For example, Patent Literature 1 discloses alumina powder for a toner additive defined by shape, particle diameter and particle size distribution used as an electrophotographic developer. It is mentioned in paragraph [0035] of the specification of Patent Literature 1 that an electrophotographic developer having no drum filming, improved image quality and excellent durability can be obtained by using the alumina powder.
Patent Literature 2 discloses a toner for nonmagnetic single-component development containing alumina particles having an Al2O3 content of 90% or more by mass attached on the surfaces of toner particles. It is mentioned in paragraph [0029] of the specification of Patent Literature 2 that the toner using the alumina particles has image characteristics of little fog, excellent reproducibility of a solid patterned image with 100% image density and excellent transferability.
Patent Literature 3 discloses a toner obtained by externally adding alumina fine particles having a specific purity, primary particle diameter and ion content to colored particles having a specific shape and particle size distribution. Examples in the specification of Patent Literature 3 disclose the evaluation on image fog, transfer failure, charging failure and image striping of images printed out using the toner.
It is also known to use metallic compounds other than metal oxides as external additives. For example, Patent Literature 4 discloses a developer for electrostatic image development obtained by externally adding cubic calcium carbonate to coloring particles. It is mentioned in pages 14 to 15 of the specification of Patent Literature 4 that the cleaning property and transferability are excellent, no filming occurs on a photosensitive member, and an image having no fog and blur can be formed by the developer.
Patent Literatures 5 and 6 disclose an alumina fine particle having an aspect ratio of 3 or less is excellent in dispersibility.